Post game refers to defeating the Elite 4. You have to have become the Kalos Region champion before you can find a lot of the Mega Stones, as well as entering certain locations containing legendary Pokemon.

Think original Mewtwo in the cave behind Cerulean City -- there's a guy blocking it every time you go there until after you defeat Gary, at which point he will have vanished, giving you access to the cave. In more recent games, this has been applied to a lot of Legendaries, special items, extra quantities of previously extremely limited items, etc.